It is a common industrial practice to encapsulate a wire core such as electrical wires, cables, optical fibers, filaments, etc. with a jacket or sheathing to insulate or isolate them from the ultimate environment in which they are to be placed. With today's technologies, the insulation is usually of a polymer plastic composition that is supplied from an extruder in a molten state and dispensed onto the wire core via a crosshead through which the core is being continuously fed. In the course of advancing to the dispensing site, the molten plastic enters a flow path provided by the crosshead that is intended to afford concentric and uniform distribution about the core prior to it emerging into the ambient atmosphere. Essential in the application of such sheathing is obtaining uniformity of thickness and the absence of voids by preventing irregularities that can readily occur. In some applications, the formed sheathing is water cooled before being coiled or otherwise stored for shipment.